The present invention relates to a controllable damping force hydraulic shock absorber which is mounted on a suspension apparatus of a vehicle such as an automobile.
As a hydraulic shock absorber mounted on a vehicle such as an automobile, there is known a controllable damping force hydraulic shock absorber in which damping force characteristics can be appropriately controlled according to road surface conditions, vehicle running conditions, etc., so as to improve ride comfort and steering stability.
Generally, a controllable damping force hydraulic shock absorber is arranged as follows. A piston, which has a piston rod connected thereto to form a piston assembly, is slidably fitted into a cylinder in which a hydraulic fluid is sealably contained. The piston divides the inside of the cylinder into two chambers. The piston assembly is provided with a main hydraulic fluid passage and a bypass passage which permit communication between the two chambers in the cylinder. The main hydraulic fluid passage is provided with a damping force generating mechanism including an orifice and a disk valve, and the bypass passage is provided with a damping force control valve for changing a flow path area of the bypass passage.
In this arrangement, a small damping force is generated by opening the bypass passage through the damping force control valve so as to reduce a resistance to the flow of the hydraulic fluid between the two chambers in the cylinder. On the other hand, a large damping force is generated by closing the bypass passage so as to increase the flow resistance between the two chambers. Thus, damping force characteristics are controlled by opening/closing the bypass passage through the damping force control valve.
However, when a damping force is controlled only by changing the flow path area of the bypass passage, the following problem arises. That is, although damping force characteristics can be changed to a large extent in a low piston speed region in which a damping force is dependent on the restriction of an orifice in a fluid passage, damping force characteristics cannot be greatly changed in intermediate and high piston speed regions in which a damping force is dependent on the degree of opening of the damping force generating mechanism (disk valve) in the main hydraulic fluid passage.
As a countermeasure, there is known a controllable damping force hydraulic shock absorber, as disclosed in Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 7-332425 (corresponding to U.S. Pat. No. 5,655,633), in which a pilot type damping force control valve is provided as a damping force generating mechanism in each of a main hydraulic fluid passage for an extension stroke and a main hydraulic fluid passage for a compression stroke. In this pilot type damping force control valve, a back-pressure chamber (a pilot chamber) is formed at the back of a disk valve. The back-pressure chamber is communicated through a fixed orifice with a cylinder chamber on the upstream side of the disk valve and communicated with a cylinder chamber on the downstream side of the disk valve through a flow rate control valve (a pilot control valve).
In this controllable damping force hydraulic shock absorber, the flow rate control valve is operated, to thereby change the flow path area of a flow passage between the two chambers in the cylinder, while the pressure in the pilot chamber is changed due to a pressure loss at the flow rate control valve, to thereby change the valve-opening pressure of the disk valve. Thus, orifice characteristics (in which a damping force is approximately proportional to the square of the piston speed) and valve characteristics (in which a damping force is approximately proportional to the piston speed) can be controlled simultaneously, thus making it possible to control damping force characteristics within a wide range.
Further, there is known a controllable damping force hydraulic shock absorber disclosed in Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 2001-12530 (corresponding to U.S. Pat. No. 6,371,262), in which a pilot type damping force control valve is provided in each of an extension-stroke passage and a compression-stroke passage, and a damping force for an extension stroke and a damping force for a compression stroke are directly controlled by means of pilot control valves of a pressure control type.
However, in the above-mentioned conventional controllable damping force hydraulic shock absorbers using pilot type damping force control valves, the following problems arise. Namely, when a valve body of a pilot control valve for an extension stroke and a valve body of a pilot control valve for a compression stroke are integrally formed as a single member so that they are operated by means of a single actuator, the single member usually takes a form of spool valve in the case where the valves function as flow rate control valves. In such a case, the flow rate of the hydraulic fluid is controlled by changing the longitudinal stroke position of the spool to change the flow path area. In this arrangement, from the viewpoint of accuracy of control of the actuator, it is necessary to obtain a substantially long stroke of the spool valve between a position for a minimum flow path area and a position for a maximum flow path area for both an extension stroke and a compression stroke. Consequently, a solenoid used as the actuator is required to have a size sufficient for obtaining a long stroke of the spool valve, thus making it difficult to reduce the size of solenoid. In the case of using pressure control valves as the pilot control valves, damping characteristics of the controllable damping force hydraulic shock absorber relative to a current applied to the solenoid are determined, based on set loads and spring constants of two springs which are provided on opposite sides of the valve bodies with respect to the direction of stroke thereof and which impart bias forces to the valve bodies. Therefore, adjustment of a set load of a spring must be effected, with respect to each of the two springs, in consideration of tolerances of the parts or components affecting the two springs. However, in the above-mentioned Kokai No. 2001-12530, adjustment of a set load of a spring can be effected only on one side of the valve body, and individual adjustments of the set loads of the two springs cannot be effected. Therefore, fine adjustment of a set load of a spring cannot be made. Even when the arrangement is changed so that individual adjustments of the two springs can be effected, the bias forces of the two springs affect each other, so that delicate adjustment is required to achieve optimum setting of the set loads of the springs, thus making it difficult to manufacture the controllable damping force hydraulic shock absorbers on a mass production basis.
In view of the above, the present invention has been made. It is an object of the present invention to provide a controllable damping force hydraulic shock absorber utilizing pilot type damping force control valves, which is compact in size and capable of controlling a damping force within a wide range.
In order to achieve the above-mentioned object, the present invention provides a controllable damping force hydraulic shock absorber comprising: a cylinder in which a hydraulic fluid is sealably contained; a piston slidably fitted in the cylinder; a piston rod having one end connected to the piston and the other end extended to the outside of the cylinder; an extension-stroke passage which allows flow of the hydraulic fluid during an extension stroke of the piston rod; and a compression-stroke passage which allows flow of the hydraulic fluid during a compression stroke of the piston rod. An extension-stroke pilot type damping force control valve is provided so as to control the flow of the hydraulic fluid in the extension-stroke passage due to the effect of a pressure in a pilot chamber, to thereby generate a damping force. A compression-stroke pilot type damping force control valve is provided so as to control the flow of the hydraulic fluid in the compression-stroke passage due to the effect of a pressure in a pilot chamber, to thereby generate a damping force. Each of the extension-stroke pilot type damping force control valve and the compression-stroke pilot type damping force control valve includes a pilot control valve provided therein so as to control the hydraulic fluid in the pilot chamber. Either one of the pilot control valve provided in the extension-stroke pilot type damping force control valve and the pilot control valve provided in the compression-stroke pilot type damping force control valve comprises a flow rate control valve, which is adapted to control a flow path area of a flow passage connected to the pilot chamber to a desired flow path area by means of an actuator. The other pilot control valve comprises a pressure control valve, which is adapted to control the pressure in the pilot chamber to a desired pressure by means of an actuator.
By this arrangement, a damping force is controlled by controlling the pilot pressures of the extension-stroke and compression-stroke pilot type damping force control valves by means of the flow rate control valve and the pressure control valve.
In the present invention, a valve body of the pilot control valve provided in the extension-stroke pilot type damping force control valve and a valve body of the pilot control valve provided in the compression-stroke pilot type damping force control valve may be integrally formed to enable reverse characteristics of damping force to be obtained, the valve bodies being adapted to be operated by a single actuator.
By this arrangement, reverse characteristics of damping force can be obtained by using a single actuator.
The controllable damping force hydraulic shock absorber of the present invention may be arranged, such that the pilot control valve provided in the extension-stroke pilot type damping force control valve comprises the pressure control valve and the pilot control valve provided in the compression-stroke pilot type damping force control valve comprises the flow rate control valve.
Generally, in a controllable damping force hydraulic shock absorber, there is a tendency for a damping force for a compression stroke to be controlled within a narrow range as compared to a damping force for an extension stroke, due to a difference in pressure-receiving area of the piston between an extension stroke and a compression stroke. With the above-mentioned arrangement, damping force characteristics can be controlled within a wide range even during a compression stroke.
The foregoing and other objects, features and advantages of the present invention will become apparent from the following detailed description and appended claims taken in connection with the accompanying drawings.